Remove the left-hand (as looking at the front) push-button board by pressing the detent. Turn the P.B. board endwise, and stick into the cabinet.
Unplug the leads to the right-hand P.B. board from the rear of the chassis. Mark which colored leads go where.
Unplug speaker and wavemagnet antenna.
Remove chassis mounting screws from underneath the chassis shelf. You should be able to slide the chassis out.